Draghead with concentric hollow cylinders having alignable ports

ABSTRACT

A drag and draghead in which the draghead comprises concentric hollow cylinders having alignable ports, one of which being rotatable with respect to the other to change the alignment between the ports to vary the overall intake area to improve start-up characteristics and limit cavitation. A water jet arrangement is provided adjacent the intake ports to loosen the spoil and improve the intake of the drag. The drag pipe includes flexible joints and control lines to sweep it about the area to be dredged.

United States Patent Lovelace 51 Apr.25,1972

[54] DRAGHEAD WITH CONCENTRIC HOLLOW CYLINDERS HAVING ALIGNABLE PORTS [72] Inventor: Richard S. Lovelace, Cos Cob, Conn.

[73] Assignee: National Bulk Carriers Inc., New York,

[22] Filed: July 11, 1969 [21] App]. No.: 845,932

Related U.S. Application Data [63] Continuation of Ser. No. 514,025, Dec. 15, 1965,

abandoned.

[52] U.S. Cl ..37/63, 37/72, 302/15, 302/58 [51] Int. Cl. ..E02f 3/92 [58] Field ofSearch ..37/61-63,72,58, 37/59; 302/58, 15

[56] References Cited UNITED STATES PATENTS 528,977 11/1894 Pike ..37/58 530,829 12/1894 Duckham 302/58 548,242 10/1895 Wood et al ..37/72 X 811,275 1/1906 Cole ..37/62 814,019 3/1906 Clark ..37/58 842,364 l/1907 White ..37/6l 906,234 12/1908 Jackson ..37/58 1,530,654 3/1925 Daley 37/63 X 1,688,109 10/1928 Berry ...37/63 2,252,803 8/1941 Durepaire.. ..37/63 2,711,598 6/1955 Craggs, Jr. ..37/62 FOREIGN PATENTS OR APPLICATIONS 561,202 5/1944 Great Britain ..37/63 Primary ExaminerRobert E. Pulfrey Arsistan! Examiner-Clifford D. Crowder Attorney-Donald P. Gillette [57] ABSTRACT A drag and draghead in which the draghead comprises concentric hollow cylinders having alignable ports, one of which being rotatable with respect to the other to change the alignment between the ports to vary the overall intake area to improve start-up characteristics and limit cavitation. A water jet arrangement is provided adjacent the intake ports to loosen the spoil and improve the intake of the drag. The drag pipe includes flexible joints and control lines to sweep it about the area to be dredged.

17 Claims, 8 Drawing Figures PATENTED 25 m2 3, 657, 829

SHEET 10F 5 [II I i I INVENTOR.

' RICHARD S. LOVELACE M WWW ATTORNEY PKJEFTEQHR 2 5 ma SHEET 2 0F 5 INVENTOR. RICHARD S. LOVELACE WW ATTORNEY 1 ".??322'1'???Hr: 25 m2 3 657, 829

FIG. 5

Ill

Ill

III

FIG. 6

INVENTOR. RICHARD 5. LOVELACE ATTORNEY DRAGI-IEAD WITH CONCENTRIC HOLLOW CYLINDERS HAVING ALIGNABLE PORTS This application is a continuation of application Ser. No. 14,025, filed Dec. I5, 1965, now abandoned.

This invention relates to a drag and draghead for dredging solid material from the bottom of a body of water such as a river, bay, or even the open sea; and particularly to a draghead capable of fitting into and picking up material from narrow underwater trenches and other depressions.

One of the principle objects of this invention is to provide a draghead capable of efficiently removing a relatively large quantity of solid material in a given time, but which is also capable of penetrating relatively narrow crevices in the bedrock on the bottom. The ability to penetrate narrow crevices requires that the draghead have a small cross-sectional area, which normally would limit the total quantity of solid material that could be drawn through the draghead in a given time, and therefore means are provided in the draghead and in the drag as a whole to increase the effective area covered. The draghead is a basically cylindrical structure having openings at its end and along its side walls that can be selectively opened and closed by means of a concentric cylinder with properly spaced apertures capable of rotating with respect to the other cylinder to provide openings of the proper size to draw in solid material and water in the proper ratio for efficient operation.

It is another object of the present invention to provide for the dredging of a channel of considerable greater width than the width of the draghead itself. In order to achieve this object, several lines are attached to the drag to control its location with respect to the hull of the dredge to which it is attached. While the lines may interact to some degree, some of them primarily control the angle that the drag makes with respect to the horizontal and others primarily control the position of the drag transverse to the direction of motion of the dredge. By means of the latter lines, together with suitable winches and booms, the drag may be moved from side to side to sweep an area of the bottom. In order to have this freedom of motion, the drag includes flexible joint means which also provide some flexibility to keep the mechanism from being damaged if it strikes the bottom because of wind or wave action or deliberate movement of the dredge along its path of operation.

Further objects may be determined from studying the following specification together with the drawings, in which:

FIG. 1 shows a portion of a dredge including the drag and operating lines and booms for the drag;

FIG. 2 shows a cross-sectional view of the dredge of FIG. 1 with the drag in operating position;

FIG. 3 is a cross-sectional view of the draghead of FIGS. 1 and 2;

FIG. 4 is a cross-sectional view of the draghead of FIG. 3 along the lines 4-4;

FIG. 5 is a cross-sectional view of the draghead of FIG. 3 along the lines 5-5;

FIG. 6 is a cross-sectional view of the draghead of FIG. 3 along the lines 6-6;

FIG. 7 is a developed view of the inner cylinder of the draghead in FIG. 3; and

FIG. 8 is a developed view of the outer pipe of the draghead.

The dredge of FIGS. 1 and 2 comprises a hull 11 with a drag l2 controlled in its side to side location by lines 13 and 14. The drag comprises a trunnion 16 and two flexible joints 17 and 18, of which the joint 17 is shown in the present embodiment as a ball joint and the joint 18 as a flexible section, although their positions may be reversed or they may both be flexible sections or ball joints. Below the flexible section 18 is a straight length of the dragpipe 19 divided, in this embodiment, into several sections at the end of which is located a draghead 21 which will be described in greater detail hereinafter.

The line 13 may be referred to as the drag outhaul line since, as shown particularly in FIG. 2, it extends outwardly of the dredge 11 and runs through a sheave 22 at the end of a boom 23. The other line 14 is referred to as the drag inhaul line and runs to another sheave 24 at the bottom of a hollow well 25 that passes up through the hull of the dredge 11, although it could extend up along the outside of the hull. The drag inhaul line 14 also passes through this same well to a storage reel 26 operated by a motor and stalled torque converter 27. The drag outhaul line 13 runs the length of the boom 23 and terminates at a winch 28 controlled by a motor 29. The well 25 has a gate valve 30 at its lower end to permit it to be sealed off from the sea for inspection and service.

In operation, the drag 12 extends downward and to the rear at an angle of about 45. Ideally the dredge 11 should move forward slowly guided by lines fore and aft and held by anchors on each side. The fore and aft lines may be attached to anchors also, but these will normally be quite far from the dredge, whereas the side anchors may be relatively close. As each spot along the bank 33 is dredged to the depth of effective operation of the draghead 21, which is a depth of about 10 feet in the present embodiment, the side anchors are moved forward slightly to permit the vessel to inch ahead.

The draghead at any given time has only a limited area of the bank 33 from which it can draw material, and in order to reduce the number of passes that the dredge 11 must make to cover a large region, the drag 12 is swept from side to side by alternately drawing in and paying out the outhaul line 13 and the inhaul line 14. As may be seen in FIG. 1, these lines are more or less in a plane common to both of them and to the drag 12 to minimize up and down movement of the drag as it sweeps.

The ideal progression of the dredge 11 in a straight line on a smooth sea having a smooth bottom at a fixed depth will seldom be encountered for any extended period of time. Therefore, the dredge 11 may require stabilization devices to control roll and may also require means, which are known in the art, to compensate for wave action, tides and other factors that affect the position of the drag 12. Such means may control the angle that the drag makes with respect to the horizontal by lifting the end of the drag 12 up and down by a line 34 attached to a lifting eye 36 on the draghead 21.

Not only is the dredge subject to pitch, roll, and vertical movements, it may even be driven backward by adverse winds or currents. The motions may combine to slam the draghead 21 against the bottom or, at times, to drive the drag forward and upward more or less parallel to the axis of the dragpipe 19. This brings into operation the flexibility afforded by the joints l7 and 18 that permit the drag to fold up slightly, enough to prevent damage to the draghead 21 until it can be hauled out of the way by the line 34.

Within the hull of the dredge 11 is a pump 31 connected to the upper end of the dragpipe 19 to provide the necessary suction to draw material into the draghead. The output of the pump may be directed to containers or holds for treatment and then pumped out through a discharge pipe 32, or the pump 31 may be connected directly to the pipe 32 to dispose of the spoil at some distance from the dredge 11.

The draghead 21 is shown in greater detail in FIG. 3. The two major parts of the draghead are an outer cylindrical casing 51 and a hollow inner cylinder 52 concentric with the casing 51 and fitting reasonably closely therein but with sufficient spacing to permit the cylinder 52 to rotate with respect to the cylinder 51. In this embodiment, the cylinder 52 is a relatively thin-walled structure with a tapered section 53 at one end thereof. The cylinder 52 also has four bearings 54-57 spaced along its length to permit the inner cylinder to rotate easily within the outer cylinder 51. The bearings 54, 55 and 57 support the inner cylinder 52 radially while the fourth bearing 56 is a thrust bearing comprising the main flange 59 held in place between a shoulder 61 and a nut 62 threaded on the cylinder 52 and guided between bearing segments 63 and 64 within an annular chamber 66 in the outer cylinder 51.

The outer cylinder is divided into several shorter sections starting with a tapered section 67 at the same end as, and

fitting closely around the tapered section 53 of the inner cylinder, which is the lower end or entrance end when the draghead is in operation. The section 67 also includes the outer race of the bearing 54 to provide mutual, freely rotatable support of the tapered ends of the two cylinders 51 and 52 and a flange 68 joining the end section 67 to a second section 69. Appropriate seals 71-73 are placed in the sections 67 and 69 to seal off the space in which the bearing 54 is located to keep abrasive sediment out of that region and to retain the bearing lubricant. The section 69 has, for part of its circumference, a markedly greater diameter than the outer diameter of the cylinder 52 to provide a chamber through which jetting water can flow, as will be described in greater detail hereinafter. The other end of the section 69 is joined by a third section 74 within which the bearing 55 is located. As in the case of the bearing 54, the bearing 55 is sealed off by seals 75-77 in the two sections 69 and 74, and the section 74 has an enlarged diameter over part of its length to provide a channel for jetting water.

At the other end of the section 74 from the section 69 is a radially outwardly extending flange 78, which is joined to another flange 79 in a fourth section 81 of the outer cylinder 51. The section 81 includes a housing 82 within which is located a motor 83 capable of underwater operation and a gear reducer 84 that drives a ring gear 86 rigidly attached to an inner cylinder 52 to rotate the inner cylinder with respect to the outer cylinder when the motor 83 is operated. The chamber 66 within which the thrust bearing 59 is located is at one end of the cylindrical section 81, and a short section 87 is located directly adjacent to the chamber 66, primarily for the purpose of housing the bearing 57. This section 87 is attached to another short section 88 within which are seals 89 and 91 that seal one end of the space within which the bearings 56 and 57 and the motor 83 and gear reducer 84 are located. The other end of this space is sealed by seals 92 and 93. At the end of the short section 88, which will be referred to as the upper end of the draghead, there is a flange 94 to be attached to a facing flange 96 at the lower end of the dragpipe 19.

Outside of the outer cylinder 51 is a smaller pipe 97 that serves as a manifold to supply jetting water to appropriate places along sections 67, 69 and 74. The jetting water to be supplied in the region of the tapered section 67 is carried through the pipe 98 at the end of the manifold 97, which is also shown in FIG. 4. As may be seen in FIG. 4, the pipe 98 forms a collar partially around the section 67 and has restricted openings 99 and 101 and its ends and similar openings 102 and 103 spaced slightly back from the ends. Water forced through the pipe 98 will emerge in direct jets in each of these openings to assist in loosening the sand, gravel, or other material to be picked up by the draghead.

Two short pipes 104 and 1040 join the main jetting pipe 97 to the cylindrical section 69. As may be seen in FIG. 3, the section 69 has inlets 105 and 105a with which the pipes 104 and 104a connect and which lead into interior spaces, or channels, 106 and 1060 which, in the upper part of the section 69, are not walled off from each other but which are walled off at the part that faces downwardly. The interior construction of this part of the section 69 is shown in FIG. in which the sector 107 comprises an interior rear wall of the jetting channels and the sector 108 is an interior wall of the enlarged portion of the cylindrical section 69. The lower ports of the channels 106 and 1060 also have front walls 109 and 109a and between the front and rear walls are several orifices 111 and 112 through which jetting water can escape into the region adjacent to the downwardly facing portion of cylindrical section 69.

The short section 74 of the cylinder 51 has its own set of apertures 114 and its own jetting water channel 106b with an inlet 10517 connected to the main jetting manifold 97 by a short pipe l04b. The annular jetting channel 106b is walled off by a front wall only in the lower portion thereof, just as in the case of the channels in the cylindrical section 69 and jetting water emerges from the set of orifices 114 in the lower section of the jetting channel.

FIGS. 7 and 8 are developed, simplified layouts of the cylinders 52 and 51, respectively, showing the apertures in both of these cylinders. In the inner cylinder 52 there are three, equally spaced openings 1l7-117b in the tapered end section 53. A center line 118 is shown through the middle of the layout in FIG. 7 merely to indicate an arbitrarily chosen center of the layout. This is not the axis of the cylinder 52 in its three dimensional form, although it would be in the same plane as that axis.

The center line 118 passes through the centers of a series of broad openings 119-122. Directly in line with and below the aperture 119 is another opening 119a, and below the opening 120 is a corresponding opening 120a. These openings 119a and 120a are, in the three dimensional form of the cylinder 52, spaced l20 from the position of the openings 119 and 120.

In the layout of FIG. 8 the flange and water channels have been omitted and all that remains is a series of apertures indicating the location of water and slurry passages through the wall of the cylinder. As in the case of the layout in FIG. 7, the layout in FIG. 8 includes a center line 123 through the middle of the developed cylinder and two other broken lines 124 and 125 spaced from the center line 23 by a distance equivalent to one-third of the circumference of the cylinder. In addition, the center line 123 corresponds to the line facing substantially vertically downward when the drag 12 is in operating position.

At the constricted end of the cylinder in the portion 67 are two apertures 126 and 126a so positioned and of such a size that any of the openings 117 and 117a and ll7b can overlap the apertures 126 and 126a. Along the cylindrical portion of the cylinder 51 are four sets of apertures 127-130 spaced to fit in between the channels 106. There are additional apertures 127a-130a on one side of the center line 123 and apertures 127b-130b spaced on the other side of the apertures 127-130. The total size of each of the sets of three apertures 127-127b through -13012 is about the same size as, or slightly smaller than, the corresponding openings 119-122 in the inner cylinder 52. Finally there is an additional set of apertures 127c-127e angularly spaced 120 from the apertures 127-127b. These apertures 127c-127e are longitudinally spaced at the same point along the cylinder 51 as the apertures 127-127b and are of the same size so that the openings 119 and 119a can overlap them, depending on the angular orientation between the outer cylinder 51 and the inner cylinder 52.

The inner cylinder 52 can be aligned with respect to the outer cylinder 51 in three basic angular positions. In the full speed dredging position, the orientation may be visualized as if the developed view of the inner cylinder in FIG. 7 were laid directly over the developed view of the outer cylinder in FIG. 8 with the center line 118 of FIG. 7 directly over the center line 123 in FIG. 8. In such a position, the apertures 126-126a of the outer cylinder 51 would be directly in line with the opening 117 at the constricted end of the inner cylinder, the apertures l27-127b would be in line with the opening 119 of the inner cylinder, the apertures 128-128b would be in line with the opening 120 of the inner cylinder, the apertures 129-129b would be in line with the opening 121 of the inner cylinder, and the apertures 130-130b would be in line with the aperture 122. In this position none of the apertures in the outer cylinder 51 except the apertures 127c-127e would be covered up and the draghead 21 would be able to pick up a maximum amount of slurry.

In the intermediate position obtained by rotating the inner cylinder 52 l20 with respect to the outer cylinder 51 and in such a direction that the center line 118 would be directly over, and in line with, the broken line 124. In this position, the apertures 129-129b and 130-130b would be covered up, but the apertures 126 and 126a would be open because they would be in line with the opening 117a, and all of the apertures 127-127e would be open. The apertures l27-127b would be in line with the opening 119a while the apertures 127c-l27e would be in line with the opening 119. The apertures 128-12812 would also be open because they would be in line with the opening 120a.

The third position obtained by rotating the inner cylinder 52 another 120 in the same direction as before would result in covering up all of the apertures 127-127b as well as the apertures 128-128b, 129-l29b, and 130-130b. However, the apertures 127c-l27e would be in line with the opening 119a and thus would be open while the apertures 126 and 126a would be in line with the opening 11712 and thus would be open.

In the operation of the dredge 11 the drag 12 is first lowered to the bottom and the motor 83 is energized to cause the inner cylinder 52 to rotate to the third, or most closed, position. Since only the end of the cylinders and the apertures 126 and 126a adjacent to the end and the apertures 127c-127e are open, the intake of slurry is limited. The reason for having the apertures l27c-127e open is to prevent cavitation by allowing sufficient water to enter the draghead. However, the other apertures 127-l27b through 130-130b are closed to keep too much free water from flowing while digging the bank. The water entering the apertures 127c-127e reduces the high velocity of water entering the open ends of the cylinders and also dilutes the slurry.

As the solid material on the bottom is drawn into the end of the draghead, the draghead sinks down into the material to an extent that makes it proper to rotate the inner cylinder 52 to the second position 120 away from the first position. In the second position, the apertures 127-1271) and 128-128b are also open to pick up slurry. Since this open area faces the bottom from which the solid material is to be drawn, the inflow into the draghead will be direct, thus permitting the draghead to dig even more deeply into the loose material.

As the draghead approaches its full depth of about feet, the inner cylinder can be rotated another 120 to the final, wide-open position in which only the apertures 127c-127e would be closed.

What is claimed is:

l. Dredging apparatus comprising a first hollow cylinder having a plurality of longitudinally spaced apertures therein; a

second hollow cylinder fitting relatively closely over said first cylinder and having a different number of apertures than said first cylinder and alignable in predetermined order with said apertures of said first cylinder; driving means engaging both of said cylinders to produce relative rotation between said cylinders to change the alignment between the apertures of said first cylinder and the apertures of said second cylinder whereby in a first position a first set of apertures near one end of said cylinders is aligned and in a second position additional apertures longitudinally adjacent said first set but spaced therefrom toward the other end of said cylinders are aligned; and attachment means at the other end of one of said cylinders to attach said apparatus to a dragpipe.

2. The apparatus of claim 1 in which said attachment means is aligned with both of said cylinders.

3. The apparatus of claim 1 in which both of said cylinders are open at said one end.

4. The apparatus of claim 3 in which both of said cylinders are tapered to a smaller diameter at said open end than elsewhere.

5. The dredging apparatus of claim 1 in which said apertures in said second cylinder are situated in sets axially spaced apart by the same distance as said apertures in said first cylinder, at least some of said sets containing unequal numbers of apertures angularly spaced to overlap said apertures in said first cylinder.

6. The dredging apparatus of claim 5 wherein different numbers of said sets of apertures of said second cylinder are alignable with different numbers of said apertures in said first cylinder in a plurality of relative angular positions of said cylinders.

7. The dredging apparatus of claim 6 in which in one of said relative angular positions of said cylinders, each of said sets of apertures in said second cylinder in a longitudinal row over laps a corresponding one of said apertures in said first cylinder. I I

8. The dredging apparatus of claim 7 in which in a second of said relative angular positions of said cylinders, each of a plurality of said sets of apertures in said second cylinder overlaps corresponding apertures in said first cylinder.

9. The dredging apparatus of claim 8 in which in a third of said relative angular positions of said cylinders, at least a pair of said sets of apertures in said second cylinder overlaps corresponding apertures in said first cylinder.

10. The dredging apparatus of claim 6 in which in each of said relative angular positions of said cylinders, at least one of said sets of apertures in said second cylinder overlaps one of said apertures in said first cylinder.

11. The dredging apparatus of claim 1 comprising: a hull, to which said apparatus is attached; a dragpipe on which said first and second cylinders are mounted; and guide means on said dragpipe to sweep it about the area to be dredged.

12. The apparatus of claim 11 in which said guide means comprises: a boom, extending laterally from said hull; a drag outhaul line supported by said boom and attached to said dragpipe near its free end; a drag inhaul line on the opposite side of said dragpipe from said drag outhaul line, attached between said dragpipe and said hull; and flexible joints in said dragpipe to permit it to sweep from side to side as said inhaul and outhaul lines are alternately drawn in and paid out.

13. The dredging apparatus of claim 12 in which said hull comprises an open, substantially vertical pipe and said inhaul line passes upwardly therethrough.

14. The dredging apparatus of claim 12 in which said boom and drag lines are substantially coplanar with said dragpipe, and form a substantially triangular pattern.

15. The dredging apparatus of claim 11 in which the drag angle of said dragpipe with respect to said hull is 45.

16. Dredging apparatus comprising a hollow inner cylinder having a first plurality of longitudinally aligned apertures spaced at specific longitudinal locations along its length and additional longitudinally aligned apertures spaced at the two of said longitudinal locations closest to one end of said cylinder but angularly displaced from said first plurality of apertures; a hollow outer cylinder co-axial with said inner cylinder and fitting relatively closely thereabout and having a second plurality of apertures along its length alignable with said first plurality of apertures of said inner cylinder in one relative position of said cylinders, some of said second plurality of apertures being alignable with said additional apertures of said inner cylinder in a second relative position; first gear means attached to said inner cylinder; a motor attached to said outer cylinder and comprising second gear means engaging said first gear means to rotate said inner cylinder with respect to said outer cylinder to change the alignment between the apertures of said inner cylinder and the apertures of said outer cylinder; a plurality of hollow ring sections on said outer cylinder; means connected to said hollow ring sections to supply water thereto; jet orifices in said hollow ring sections to direct water therefrom into regions adjacent to the apertures in said outer cylinder; and a dragpipe connected to the other end of said outer cylinder to form an extension thereof to receive material passing through the aligned apertures of said inner and outer cylinders.

17. The dredging apparatus of claim 11 in which said outer cylinder comprises a plurality of annular channels longitudinally spaced between said apertures of said outer cylinder to provide relatively narrow channels for said water, each of said channels comprising a plurality of jet orifices to direct water therefrom into regions outside of said outer cylinder and adjacent to intake areas formed by alignment of said apertures in said inner and outer cylinders. 

1. Dredging apparatus comprising a first hollow cylinder having a plurality of longitudinally spaced apertures therein; a second hollow cylinder fitting relatively closely over said first cylinder and having a different number of apertures than said first cylinder and alignable in predetermined order with said apertures of said first cylinder; driving means engaging both of said cylinders to produce relative rotation between said cylinders to change the alignment between the apertures of said first cylinder and the apertures of said second cylinder whereby in a first position a first set of apertures near one end of said cylinders is aligned and in a second position additional apertures longitudinally adjacent said first set but spaced therefrom toward the other end of said cylinders are aligned; and attachment means at the other end of one of said cylinders to attach said apparatus to a dragpipe.
 2. The apparatus of claim 1 in which said attachment means is aligned with both of said cylinders.
 3. The apparatus of claim 1 in which both of said cylinders are open at said one end.
 4. The apparatus of claim 3 in which both of said cylinders are tapered to a smaller diameter at said open end than elsewhere.
 5. The dredging apparatus of claim 1 in which said apertures in said second cylinder are situated in sets axially spaced apart by the same distance as said apertures in said first cylinder, at least some of said sets containing unequal numbers of apertures angularly spaced to overlap said apertures in said first cylinder.
 6. The dredging apparatus of claim 5 wherein different numbers of said sets of apertures of said second cylinder are alignable with different numbers of said apertures in said first cylinder in a plurality of relative angular positions of said cylinders.
 7. The dredging apparatus of claim 6 in which in one of said relative angular positions of said cylinders, each of said sets of apertures in said second cylinder in a longitudinal row overlaps a corresponding one of said apertures in said first cylinder.
 8. The dredging apparatus of claim 7 in which in a second of said relative angular positions of said cylinders, each of a plurality of said sets of apertures in said second cylinder overlaps corresponding apertures in said first cylinder.
 9. The dredging apparatus of claim 8 in which in a third of said relative angular positions of said cylinders, at least a pair of said sets of apertures in said second cylinder overlaps corresponding apertures in said first cylinder.
 10. The dredging apparatus of claim 6 in which in each of said relative angular positions of said cylinders, at least one of said sets of apertures in said second cylinder overlaps one of said apertures in said first cylinder.
 11. The dredging apparatus of claim 1 comprising: a hull, to which said apparatus is attached; a dragpipe on which said first and second cylinders are mounted; and guide means on said dragpipe to sweep it about the area to be dredged.
 12. The apparatus of claim 11 in which said guide means comprises: a boom, extending laterally from said hull; a drag outhaul line supported by said boom and attached to said dragpipe near its free end; a drag inhaul line on the opposite side of said dragpipe from said drag outhaul line, attached between said dragpipe and said hull; and flexible joints in said dragpipe to permit it to sweep from side to side as said inhaul and outhaul lines are alternately drawn in and paid out.
 13. The dredging apparatus of claim 12 in which said hull comprises an open, substantially vertical pipe and said inhaul line passes upwardly therethrough.
 14. The dredging apparatus of claim 12 in which said boom and drag lines are substantially coplanar with said dragpipe, and form a substantially triangular pattern.
 15. The dredging apparatus of claim 11 in which the drag angle of said dragpipe with respect to said hull is 45*.
 16. Dredging apparatus comprising a hollow inner cylinder having a first plurality of longitudinally aligned apertures spaced at specific longitudinal locations along its length and additional longitudinally aligned apertures spaced at the two of said longitudinal locations closest to one end of said cylinder but angularly displaced from said first plurality of apertures; a hollow outer cylinder co-axial with said inner cylinder and fitting relatively closely thereabout and having a second plurality of apertures along its length alignable with said first plurality of apertures of said inner cylinder in one relative position of said cylinders, some of said second plurality of apertures being alignable with said additional apertures of said inner cylinder in a second relative position; first gear means attached to said inner cylinder; a motor attached to said outer cylinder and comprising second gear means engaging said first gear means to rotate said inner cylinder with respect to said outer cylinder to change the alignment between the apertures of said inner cylinder and the apertures of said outer cylinder; a plurality of hollow ring sections on said outer cylinder; means connected to said hollow ring sections to supply water thereto; jet orifices in said hollow ring sections to direct water therefrom into regions adjacent to the apertures in said outer cylinder; and a dragpipe connected to the other end of said outer cylinder to form an extension thereof to receive material passing through the aligned apertures of said inner and outer cylinders.
 17. The dredging apparatus of claim 11 in which said outer cylinder comprises a plurality of annular channels longitudinally spaced between said apertures of said outer cylinder to provide relatively narrow channels for said water, each of said channels comprising a plurality of jet orifices to direct water therefrom into regions outside of said outer cylinder and adjacent to intake areas formed by alignment of said apertures in said inner and outer cylinders. 